An example of this type of endoprosthesis is the part of an artificial hip joint that is used in the femur and is supposed to replace the head and neck of the femur. To seat the prosthesis firmly in the bone, the recess for the shaft must be precisely machined according to the anchoring philosophy ("formfit" or "pressfit"). In practice, it is not possible to produce the recess in the bone so precisely that the desired seat is achieved along the whole peripheral surface of the shaft.
Therefore, a new procedure is now being used in practice in which the recess is cut out using a program-controlled robot. This makes it possible to produce the recess with high precision.
However, when today's usual straight shaft prostheses, which frequently have a rectangular cross section to achieve rotational stability, are used, the problem is that no corners can be cut with a rotary cutting head. This is mainly due to the fact that the cutting head cannot have less than a certain minimum radius for reasons of stability. The cutting head is on a relatively long shaft and its diameter must be greater than the shaft diameter, so that the shaft can follow the cutting head into the recess. If the shaft diameter is too small, the shaft bends in the lateral forces, so the desired precision cannot be achieved. As a rule, the minimum cutting head radius is therefore approximately 5 mm.
If the bone is allowed to stand in the corners corresponding to the radius of the cutting head, there is a risk that the bone will split when the prosthesis is inserted with its sharp square edges. On the other hand, if the corners are completely cut out, there are hollow spaces on both sides of the corner line, in which the prosthesis shaft is not adjacent to the bone and in which not only does the prosthesis have no stop, but there is also a danger that the bone will recede there, since there is no stress on it.
The problem of the invention is to produce an endoprosthesis of the type mentioned at the beginning so that the shaft can lie on the wall of the recess on at least part of its length with its entire peripheral surface.